The present invention relates to gas laser apparatus for lithography and, more particularly, to gas laser apparatus for lithography, e.g. KrF excimer laser apparatus, ArF excimer laser apparatus, or fluorine laser apparatus
With the achievement of small, fine and high-integration semiconductor integrated circuits, it has been demanded that exposure systems for the manufacture of such highly integrated circuits be improved in resolution. Under these circumstances, the wavelength of exposure light emitted from light sources for lithography is becoming shorter, and gas laser apparatus emitting light shorter in wavelength than light emitted from conventional mercury lamps are used as light sources for semiconductor lithography. At present, KrF excimer laser apparatus that emit ultraviolet radiation of wavelength 248 nm are used as the gas laser apparatus for lithography. In addition, ArF excimer laser apparatus emitting ultraviolet radiation of wavelength 193 nm and fluorine (F2) laser apparatus emitting ultraviolet radiation of wavelength 157 nm are promising as next-generation light sources for semiconductor lithography.
In the KrF excimer laser apparatus, a mixed gas of fluorine (F2) gas, krypton (Kr) gas and a rare gas as a buffer gas, e.g. neon, is used as a laser gas, which is a laser medium. In the ArF excimer laser apparatus, a mixed gas of fluorine (F2) gas, argon (Ar) gas and a rare gas as a buffer gas, e.g. neon, is used as a laser gas. In the fluorine laser apparatus, a mixed gas of fluorine (F2) gas and a rare gas as a buffer gas, e.g. helium (He) or/and neon (Ne), is used as a laser gas. In these apparatus, the laser gas as a laser medium, which is in the form of a mixed gas, is sealed in a laser chamber under several hundred kPa and excited by generating an electric discharge in the laser chamber.
More specifically, in the laser chamber, a pair of main discharge electrodes for exciting the laser gas are disposed to face each other at a predetermined distance in a direction perpendicular to the laser oscillation direction. A high-voltage pulse is applied between the pair of main discharge electrodes. When the voltage across the main discharge electrodes reaches a certain value (breakdown voltage), a dielectric breakdown occurs in the laser gas between the main discharge electrodes, and thus a main discharge starts. The laser medium is excited by the main discharge.
Accordingly, such gas laser apparatus for lithography perform pulsed laser oscillation by repetition of the main discharge and emit pulsed laser beam. At the present state of the art, the laser pulse repetition rate is 2 kHz or higher.
Gas laser apparatus for lithography are characterized in that because the oscillation pulse width (Tis) is usually about 20 ns, the peak power of the output light is large, and that because the wavelength of the output light is short, the photon energy is high. Here, Tis is used as the laser oscillation pulse width.
Assuming that the deterioration of optical elements is caused by the two-photon absorption of laser light, the damage to the optical system is known to be in inverse proportion to Tis under the same laser energy conditions. Tis is defined byTis=[∫P(t)dt]2/∫P(t)2dt  (1)
where P(t) is the laser intensity depending upon time t.
Therefore, it is demanded that the laser pulse width Tis be stretched to achieve a longer pulse width as one method for reducing the damage to an optical system mounted in an exposure system. The achievement of a longer pulse width is also demanded from the following points of view.
In a projection exposure system, an image of a mask provided with a circuit pattern or the like is projected through a projection lens onto a work, e.g. a wafer, coated with a photoresist. The resolution R of the projected image and the depth of focus DOF are expressed byR=k1λ/NA  (2)DOF=k2λ/(NA)2  (3)                where k1 and k2 are coefficients reflecting the characteristics of the resist and so forth; λ is the wavelength of exposure light emitted from a light source for lithography; and NA is a numerical aperture.        
To improve the resolution R, the wavelength of exposure light is reduced, and the NA is increased, as will be clear from Eq. (2). However, the depth of focus DOF decreases correspondingly, as shown by Eq. (3). Consequently, the influence of chromatic aberration increases. Therefore, it is necessary to further narrow the spectral linewidth of exposure light. In other words, it is demanded that the spectral linewidth of laser beam emitted from the gas laser apparatus for lithography be further narrowed.
It is stated in Proc. SPIE Vol. 3679.(1999) 1030-1037 that according as the laser pulse width increases, the spectral linewidth of laser beam narrows. This was actually proved by an experiment conducted by the present inventors. In other words, it is demanded in order to improve the resolution R that the spectral linewidth of laser beam be further narrowed. For this purpose, it is essential to further stretch the pulse width of laser beam.
In regard to the above-described gas laser apparatus for lithography, a technique has heretofore been proposed whereby a predetermined amount of a rare gas (e.g. xenon: Xe) different in kind from the rare gas in the laser gas is added to the laser gas to improve energy stability between the pulses of emitted laser beam and to increase the laser output.
More specifically, Japanese Gazette Containing the Pat. No. 3,046,955 states that less than 100 ppm of an additional gas (e.g. less than 10 ppm of oxygen and a certain amount of a rare gas (Xe or the like) heavier than the rare gas in the laser gas) is added to the laser gas sealed in the laser chamber of a KrF excimer laser apparatus or an ArF excimer laser apparatus to improve the energy stability.
Further, it is stated in Japanese Patent Application Unexamined Publication (KOKAI) Nos. 2000-261074, 2000-261075, 2000-261082 and 2000-294856 that if a predetermined amount of xenon gas is added to a laser gas, burst characteristics and spike characteristics are improved. That is, in a burst operation in which a continuous pulse oscillating operation for a predetermined period of time and oscillation suspension for a predetermined period of time are repeated, it is possible to improve burst characteristics in which the energy is high at the beginning and gradually reduces thereafter. It is also possible to improve spike characteristics in which the energy is high at the beginning of each continuous pulse oscillating operation and gradually reduces thereafter.
As stated above, it is known that addition of xenon to the excimer laser gas allows an improvement in laser performance, e.g. energy stability. To further increase exposure systems in performance and to increase the lifetime of gas laser apparatus for lithography, however, it is demanded that the gas laser apparatus for lithography be improved to provide a higher output and to exhibit higher stability, and further, the laser pulse width of laser beam emitted therefrom be further stretched.
Under the above-described circumstances, the present inventors took notice of the temperature of the laser gas at the time of adding xenon thereto and obtained laser characteristic data under various temperature conditions. As a result, we found that the laser characteristics are highly dependent upon the temperature conditions.